Since Genyornis's extinction date has been traced to 40,000/50,000 years ago the painting probably dates to around that period, which would make in Australia's oldest known example of cave art. Since the beast's numbers declined very quickly after humans arrived in Australia, the most popular explanation for their demise is that they were hunted to death. Here's another picture:
About the same height as an ostrich, but twice as heavy, so about 500 to 700lbs.
